The present invention describes a method for the post-treatment of fully sealed anodized aluminum parts, especially for the automotive industry. An aqueous silicate solution is applied to a fully sealed anodized aluminum layer having a film thickness of at least 5 μm and a film weight of at least 13 g/m2, respectively. The solution preferably contains an alkali metal (M) silicate with not more than 2.0 wt.-% of SiO2, in which the ratio of SiO2:M2O is preferably not more than 2. This treatment increases the alkaline stability according to the standardized corrosion tests in the automotive industry without any further treatment or organic coating applied to said treated aluminum surface.
The electrochemical formation of oxide layers on aluminum is a well-known and widely used industrial procedure to produce protective and/or decorative coatings on aluminum and/or aluminum alloys. Electrolytically produced aluminum oxide layers protect the base metal from corrosion and weathering and furthermore may increase the surface hardness and the abrasive resistance of the aluminum part.
The different processes of anodizing are described briefly in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. 9 (1987), pp. 174- 176. Anodizing of the aluminum material can be accomplished by standardized methods in electrolytes such as sulfuric acid (Eloxal GS), chromic acid (Bengough-Stuart), phosphoric acid (Boeing) and oxalic acid (Eloxal GX). The Eloxal GS method applies direct current densities of 0.5-3 A/dm2 at voltages between 18-21 V and a bath temperature of 10-25° C. Through this treatment, film thicknesses of the anodized aluminum oxide layer of approximately 45 μm can be obtained, which is a maximum film thickness determined by the equilibrium of the oxide formation rate and its dissolution rate in the sulfuric acid solution at the specific process parameters chosen.
Such anodized aluminum layers are comprised of 1) a thin compact layer on top of the base metal that acts as a primary barrier coating against corrosive attack, which is only up to 2% of the overall layer thickness, and 2) a porous and amorphous oxide layer as the main constituent of the anodized layer. The porosity of the anodized layer may be favorable for the adhesion of further applied organic coatings, but exhibits a major drawback, namely the lack of protection against corrosive media rendered by the anodized aluminum. Therefore, and to impart maximum corrosion stability, the anodized aluminum layers have to be sealed in a subsequent process step. During sealing, which might be a hot sealing and/or cold sealing process, the aluminum oxide becomes hydrated and is transformed from its amorphous, essentially water-free constitution to the boehmite structure. This transformation is accompanied by a volume expansion or swelling of the oxide that in turn procures the sealing of the porous structure. Hot sealing of the anodized layer is usually performed in hot water or in steam, whereas the cold sealing process is operated at temperatures close to 30° C. in the presence of nickel fluoride. Sealing improves the corrosion resistance and resistance to weathering of anodized aluminum parts in a pH range from 5-8 (T. W. Jelinek, Oberflächenbehandlung von Aluminum, Eugen G. Leuze Verlag, 1997, ch. 6.1.3.1)
In the prior art, treatment of aluminum surfaces with silicate solutions is well known. For example, the sealing of porous anodized aluminum surfaces to increase corrosion resistance is described in U.S. Pat. No. 6,686,053. Hydrophilizing the aluminum surface in lithographic printing technologies is described in U.S. Pat. Nos. 3,181,461, and 2,714,066. In these areas of application, silicate treatment is favorable due to the strong affinity of aluminum and silicon to form a mixed oxide. Thus, aqueous silicate solutions support sealing anodized aluminum by precipitating and forming mixed oxides within the pores of the coating and in hydrophilizing aluminum oxide surfaces by the formation of thin layers comprising silicon dioxide on top of the aluminum oxide.
To improve the corrosion resistance of sealed anodized aluminum surfaces, metal complexes of zirconium- and/or titanium (EP 0193964) and dispersed particulate matter like silicon dioxide and/or aluminum oxide (EP 1064332) have been added to the aqueous silicate solution. Nevertheless, these post-treatments cannot prevent the anodic aluminum oxide film from being stripped away in corrosive environments with a high pH. This is especially the case when aluminum parts of car bodies are being exposed to detergent solutions in vehicle wash stations which might have a pH of 12.5-13.5. As aluminum gathers more importance as a construction material in the automotive industry, manufacturers have started to issue test standards (AUDI TL212, VOLVO TR31804674) to their suppliers in order to reject anodized aluminum parts with low alkaline resistance. Thus, there is a need for anodized aluminum surfaces and treatments for such surfaces that pass these alkali tests.
The post-treatment of sealed anodized aluminum with aqueous silicate solutions in order to hydrophilize the aluminum surface for lithographic printing is disclosed in U.S. Pat. No. 5,811,218. The corrosion resistance of the silicate treated anodized and sealed aluminum layer, which is a prerequisite for a metal to fulfill the standards of the automotive industry, is neither discussed nor revealed in U.S. Pat. No. 5,811,218. Due to the fact that the subject matter of this document is not related to the use of aluminum parts in the automotive industry, the aluminum oxide layers described therein are much thinner (1-8 g/m2) and the sealing time per micron much shorter (65 seconds/μm) than needed to meet the specific requirements and quality standards of the automotive industry.
EP 1625944 characterizes a silicate treatment of sealed and unsealed anodized aluminum plates for lithographic printing, which is first aimed to hydrophilize and/or seal the aluminum oxide surface, and secondly to enhance the resistance of the lithographic printing plate against dissolution by the alkaline developer. Here, a sealing ratio (SR) of the anodized aluminum layer of at least 50% is proposed before the hydrophilizing step, including the silicate treatment, can be performed. The treatment according to EP 1625944 is not sufficient to provide the alkaline and corrosion resistance that is mandatory in the automotive industry. EP 1625944 does not reveal the resistance of their layers exposed to an aqueous alkaline solution that contains corrosive agents such as halide ions.
Surprisingly, the present inventor found that treatment of a sealed anodized aluminum layer with an aqueous silicate solution according to the invention described herein provides improved alkaline stability. Specifically, an alkaline stability of the aluminum material for at least 10 minutes, preferably for at least 14 minutes and most preferably for at least 16 minutes at a temperature of 23±2° C. in a solution containing a mixture of 0.2 wt.-% sodium phosphate and 0.02 wt.-% sodium chloride and sodium hydroxide with a pH value of at least 11.5, preferably at least 12.5, but not higher than 13.5 was produced when the aluminum material was processed according to the inventive process.
Within this invention, alkaline and corrosive stability of the aluminum material is defined on the basis of a standardized testing method introduced in the automotive industry whereupon the visual appearance of the aluminum material after a defined exposure to the aforesaid alkaline testing solution that contains a mixture of 0.2 wt.-% sodium phosphate and 0.02 wt.-% sodium chloride and sodium hydroxide with a pH value of at least 11.5 is evaluated. The classification system of the standardized corrosion tests AUDI TL212 and VOLVO TR31804674 covers the following specifications of the visual appearance of the aluminum material after exposure to such a testing solution in the order of increasing corrosive damage:                Grade 0: no visible change in appearance        Grade 1: slight dulling of luster        Grade 2: light etch        Grade 3: etch of substrate        Grade 4: heavy etch        Grade 5: very heavy etch of substrateQuality results of at most Grade 2 after 16 minutes of exposure to a solution with a pH of 12.5 are considered to be sufficiently alkaline-stable according to the guidelines of AUDI TL212 and VOLVO TR31804674.        
As a part of the invention, the treatment of the sealed anodized aluminum layer with an aqueous silicate solution is applied within a sequential process of surface finishing of an aluminum material that is comprised of:                a) cleaning and/or electro-polishing and/or desmutting an aluminum material;        b) anodizing the aluminum material up to a film thickness of at least 5 μm;        c) cold sealing or hot sealing of the anodized aluminum material up to a sealing ratio (SR) of at least 90%, preferably 95%, and most preferably 99%;        d) treatment of the sealed anodized aluminum material with an aqueous silicate solutionwith or without rinsing and/or drying in between the listed process steps and with or without applying an organic coating to the aluminum after the process step d) has been accomplished.        
The scope of the invention also includes an aluminum material produced by treating the surface thereof sequentially by the following process steps:                a) anodizing an aluminum material up to a film thickness of at least 5 μm;        b) sealing of the anodized aluminum material up to a sealing ratio (SR) of at least 90%, preferably 95% and most preferably 99%;        c) treatment of the sealed anodized aluminum material with an aqueous silicate solution,whereupon the aluminum material treated in that way shows at most a light etch (Grade 2) in appearance after exposure to an alkaline testing solution with a pH value of at least 11.5, preferably at least 12.5, but not higher than 13.5 for at least 10 minutes, preferably at least 14 minutes and most preferably at least 16 minutes at a temperature of 23±2° C.        
The aluminum material according to this invention may be used in exterior applications such as a building material for window frames, doors and claddings, and preferably used in the automotive industry as a member of vehicle bodies and/or vehicle wheels.
The aluminum material used for the silicate treatment and/or within the process of aluminum surface finishing according to this invention is selected from pure aluminum containing at least 99 wt.-% aluminum or aluminum alloyed with copper, manganese, titanium, silicon, zinc and preferably magnesium where the magnesium content is preferably not more than 5 wt.-% and most preferably not more than 1 wt.-%.
Preferably, the aqueous silicate solution used according to the present invention contains not more than 2.0 wt.-% of SiO2, more preferably not more than 1.0 wt.-%, and most preferably not more than 0.5 wt.-%, but not less than 0.05 wt.-% SiO2 and more preferably not less than 0.1 wt.-%.
Furthermore, the silicate solution is preferably comprised of an alkali metal (M) silicate such as potassium silicate, lithium silicate and more preferably sodium silicate, where said aqueous solution preferably exhibits a molar ratio of SiO2:M2O, that is not more than 2, more preferably not more than 1.5, but not less than 0.5 and most preferably equals 1. The pH value does not need to be adjusted and thus may be left at the value provided by the dissolved silicate.
Optimized conditions for the silicate treatment are maintained, when said treatment is performed at a temperature of at least 40° C., preferably at least 50° C., but not higher than 90° C. and preferably not higher than 70° C., and most preferably at 60° C., and said treatment is performed for at least 10 seconds, preferably at least 80 seconds, but not more than 300 seconds, preferably not more than 160 seconds and most preferably for 120 seconds.
Furthermore, it is beneficial to the appearance of the aluminum part after the treatment according to this invention that the silicate treatment solution contains a wetting agent, preferably anionic and/or nonionic surfactants in a concentration of preferably at least 50 ppm, more preferably at least 200 ppm, but preferably not more than 1000 ppm and more preferably not more than 600 ppm.
The nonionic surfactant can be one or more selected from the group of alkoxylated, preferably ethoxylated or propoxylated, branched or straight alkyl alcohols or branched or straight arylalkyl alcohols or branched or straight fluoroalkyl alcohols or branched or straight alkyl amines or from the group of alkylpolyglycosides. The alkyl moiety of the selected nonionic surfactant consists preferably of at most 18, more preferably of at most 12, but at least 6 carbon atoms. Nonlimiting examples of suitable surfactants are sold under the trade names Triton®, Tergitol®, Merpol® and Zonyl®. The anionic surfactant can be one or more selected from the group of branched or straight alkyl or alkylaryl or alkylpolyether sulfates and/or sulfonates and/or phosphonates preferably with not more than 12 carbon atoms in the alkyl chain.